#include<stdio.h>
#include<stdlib.h>

int missingNumber(int *nums, int numsSize)
{
	int i, sum = 0, rsum = 0;
	for(i = 0; i < numsSize; i++)
	{
		sum += nums[i];
	} 
	
	for(i = 0; i < numsSize + 1; i++)
	{
		rsum += i;
	}
	
	return rsum - sum;
} 

int main()
{
	int numsSize, i, x;
	scanf("%d", &numsSize);
	
	int *nums = (int *)malloc(numsSize * sizeof(int));
	
	for(i = 0; i < numsSize; i++)
	{
		scanf("%d", &nums[i]);
	}
	
	x = missingNumber(nums, numsSize);
	
	free(nums);
	
	printf("The missing number is %d.\n", x);
	
	return 0;
}
